This invention relates to an apparatus comprising an electron multiplier for counting the number of charged particles in, for example, a mass spectrometer.
In the manner known in the art, a mass spectrometer comprises a number counting apparatus of the type described. The number counting apparatus is supplied with ions from an analyzer for use in separating the ions according to masses. The number counting apparatus is for use in counting the ions of individual masses which are supplied thereto as an ion current. Ions of a particular mass may amount to a considerable number while ions of other masses are small in number. The number counting apparatus must therefore have a wide measuring or dynamic range, for example, between 120 dB and 140 dB.
The number counting apparatus usually comprises an open ended electron multiplier for producing an electric current in response to the ion current. An amplifier is for amplifying the electric current to produce an amplified current. An ammeter or a like meter is used as a measuring device in measuring the amplified current to get the number of ions which enter the electron multiplier as the ion current per unit time.
As will later be described more in detail, a conventional electron multiplier comprises a first and a second collector in order to widen the measuring range. More particularly, the electron multiplier comprises a dynode member having an entrance and an exit end in the manner known in the art. The first collector is for collecting secondary electrons from the exit end to produce a first electric current. The second collector is for collecting secondary electrons from a dynode element which is selected between the entrance and the exit ends. The second collector produces a second electric current which is weaker than the first electric current. In other words, a sensitivity ratio of the second electric current to the first electric current is less than unity.
In a conventional number counting apparatus which comprises the double-collector electron multiplier of the above-described type, only one amplifier is used in selectively amplifying the first and the second electric currents. For the selective supply of the electric currents, a switch is interposed between the double-collector electron multiplier and the amplifier to switch the measuring range.
In the conventional number counting apparatus, the above-mentioned sensitivity ratio inevitably fluctuates. It is therefore necessary on switching the measuring range to calibrate the sensitivity ratio. In other words, it is impossible with the conventional number counting apparatus to readily switch the measuring range. Furthermore, the measuring range switching has been rendered more troublesome by the single amplifier. This is because the amplifier unavoidably has a stray capacity. When the measuring range is switched from one to the other, the ions must be counted after the stray capacity is completely discharged.